Dinuclear carbonyls

As can be seen from Table 1, a whole set of compounds is available from the reaction of Eq. (5), which shows the high variability of both the substituents at silicon and the introduced metal. This method is only limited by the steric demand of the substituents. If the residues at the silicon are too bulky, no reaction according to Eq. (5) occurs. The method also allows the use of dinuclear carbonylate anions... 

The simplest transition metal carbonyls are mononuclear of the type M(CO)x, in other words those with only one metal atom. They are hydrophobic but soluble to some extent in nonpolar liquids, such as n-butane or propane. The dinuclear carbonyls are more complex but have the same general characteristics as the mononuclear carbonyls. The carbonyls, which are or could be used in CVD, are listed in Table 3.4 with some of their properties. 

Matrix isolation has already provided a valuable insight into the behavior of dinuclear carbonyls. In the first experiments of this kind we were able (Z2) to show that, on photolysis in solid Ar, Fe2(CO)9 loses CO to form Fe2(C0)g in both bridged and unbridged forms and the behavior of these fragments was studied ... 

Another dinuclear carbonyl which presents interesting problems is ](ri C5H5)Fe(CO) ] 2 Does the photochemistry proceed exclusively through homolytic fission to produce two (13 -05 )Fe-(C0)2 radicals or by other possible routes The discussion of this reaction has involved mechanistic and synthetic studies (77), flash photolysis (78) and low-temperature photolysis (29) - the latter work, in THF or ethyl chloride at -78°C, invokes an intermediate in which the Fe-Fe direct bond is broken but the two halves of the molecule are held together by a CO bridge. Clearly such an intriguing problem merits more detailed investigations. 

Dinuclear carbonyl anions [M2(CO)8X3] were obtained through a two-electron transfer to protons by treatment of [M(CO)6]- with HX (X = Cl, OAc, OMe) (equation 82).526 The anion has Civ symmetry. The niobium atoms are heptacoordinated with four terminal carbonyl groups each and three bridging chlorides located at the vertices of an approximately equatorial... 

The reaction of RhHL3 (L = P(iso-Pr)3) with CO was thought to give Complex 7. Instead, the product formed from the reaction in dry hexane under a CO atmosphere was a binuclear Rh(0) carbonyl compound, Rh2(CO)3L3 (Complex 8, IR v(CO) 1732, 1768, and 1957 cm"1) (12). The similar reaction of RhHL3 containing bulkier phosphines produced Rh2(CO)4L2 (L = P(cyclo-CeHii)3, PPh(f-Bu)2, P(f-Bu)3). These dinuclear carbonyl compounds can be prepared by treating trans-RhH(CO)L2 with CO at room temperature. The formation of Rh2(CO)3L3 requires several steps, and a plausible sequence is proposed below. 

Dinuclear carbonyl complexes, with copper, 2, 187-188 Dinuclear chromium(III) complexes, with halides, alkyls, aryls, 5, 317... 

Again starting from the decacarbonyl dimetalate(-I) anions, besides the mononuclear [M(CO)sX] anions, we were the first to prepare the dinuclear [M2(CO)toX] species (M = Cr, Mo, W) with the respective metal having the oxidation state 0. These anions can be formally derived from the unknown dinuclear carbonyls M2(CO)n by substituting a CO ligand by X" (X = CN, I, NCS, SH, SMe, SEt, SPh). 

The mono- and dinuclear carbonyl metalates of Cr, Mo, and W, first prepared by us using liquid NHS, have also been of great importance for the synthesis of numerous neutral complexes not accessible by other routes. Special attention is drawn to the deeply colored and very unstable mono- and dinuclear paramagnetic complexes Cr(CO)sI, Cr (CO),oI, Cr(CO)5CN, Cr(CO)sNCS, Cr COl.oNCS, and Cr(CO)sSH. 

Table 16- Photolysis of Mn and Re dinuclear carbonyls in the presence of halogen donors...

The dinuclear carbonyl Fe2(CO)9 reacts with Ph2PCH2CH2SnPh3 to form a chelated complex 92 via (CO)4FePPh2CH2CH2SnPh3 the same compound is formed in the reaction of (CO)4Fe(H)SiMePh2 with Ph2PCH2CH2SnPh3 via a silyl group elimination291. 

The primary photochemical reaction of the [MM (CO)i0] (M, M =Mn, Re) complexes in solution has been shown to result in (1) homolytic fission of the metal-metal cr-bond to form M(CO)s radicals, (2) dissociation of a CO ligand to form [MM (CO)9], and (3) non-dissociative relaxation to the electronic ground state [86]. Similar behaviour is shown by other dinuclear carbonyl complexes [87]. 

The highly toxic osmium complex Os3(CO)8(M-Cl)2PPh3 was immobilized via an anchored phosphine ligand as Os3(CO)8(/x-Cl)2Ph2P-[Res]. The bonding may be more complex, for example, Kalck and coworkers have proposed that each rhodium atom of the dinuclear carbonyl is anchored to the resin (equation 9) ... 

The photochemistry of more complex carbonyls that contain ligands other than CO has been stndied in some detail. Time-resolved IR spectroscopy has been nsed to stndy the photochemistry of the dinuclear carbonyl [CpFe(CO)2]2 (see Cydopentadienyl) in solution and the results of these experiments have been compared with those obtained from earlier matrix-isolation work. ... 

Attention turned to the dinuclear carbonyls Mn2(CO)io, Re2(CO)io, and Co2(CO)g. Regarding mechanistic information, the study of Re2(CO)io has been the most fruitful. Here two distinct oxocarbonyl species, which appear to contain only one rhenium atom each, are formed. The final product of the reaction is the O-bridged oxide Re207. In a similar vein, Mn2(CO)io yields Mu207 upon matrix photooxidation, although there is no hint of any intermediate in this process, no matter what steps are taken to produce such species. The photooxidation of C02 (CO)g is likely to follow a pathway even more complex than that of Mn2(CO)io or Re2(CO)io- In this case not only is there competition between M-C and M-M bond ruptme as the primary photochemical step, but there is also the complication of photoisomerism of the molecule. 

An unexpected reaction was reported to occur on treatment of [Cu(dcmpz)] , 12, with CO in pyridine (27). The dinuclear monocarbonyl complex Cu2(dcmpz)2(py)2(CO), 37, was obtained and its X-ray crystal structure was determined. An analogy was drawn between the CO-hemocyanin adduct and the dinuclear carbonyl Complex 37. The observed stoichiometry of CO bonding... 

The alkoxocarbonyl complexes described above are thought to be the products of the nucleophilic attack of MeOH or EtOH on the dinuclear carbonyl Complexs 55 and 71, which were previously oxidized by iodine, as the latter complex are indefinitely stable in alcohols (82). 

The dinuclear carbonyls of group 7 are reduced by alkali metals in THF (equation 41). The pentacarbonylmetalate(—I) anions are appropriate entries into the hydrido- or alkyl (aryl) pentacarbonyl derivatives (see equations 42 and 43). These compounds have pseudooctahedral structures of C4 symmetry with the hydrido or alkyl (aryl) ligand occupying one of the positions within the coordination polyhedron. 

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